1. Field of the Invention
The present invention relates to a method for controlling transmission power when transmitting a radio signal from a base transceiver station to a mobile station in a mobile communications system, for example, a CDMA mobile communications system, where a mobile station (MS) is able to receive and combine signals from a plurality of base transceiver stations (BTS). In particular, it relates to a method for controlling transmission power in the middle of handover.
2. Description of the Related Art
In the above CDMA mobile communications system etc., since the same frequency bandwidth is shared by a plurality of communicating parties, the signal of one communicating party becomes an interference signal for the signal of another communicating party, so it is desirable to keep the transmission power of the radio signals used by communicating parties as low as possible.
Further, when a mobile station near to a base transceiver station and a mobile station far from the base transceiver station simultaneously communicate with the base transceiver station, if the transmission power of the uplink signals from the mobile stations are the same, the base transceiver station receives the uplink transmission signal from the near mobile station at a high power level, while receives the uplink transmission signal from the far mobile station at a low power level. Therefore, the signal from the far mobile station receives strong interference due to the signal from the near mobile station and the quality of the signal from the far mobile station received by the base transceiver station degrades to a large extent, that is, a “far/near problem” occurs. As a technique for solving this far/near problem, methods for controlling the transmission power have been studied.
Looking at the control of for example the downlink transmission power during a soft handover in a CDMA mobile communications system, if a mobile station receives signals from a first base transceiver station and second base transceiver station communicating with it, the received signals are despread, then combined and the quality of reception after combination is measured by the mobile station. For example, the ratio of the level of reception of the interference wave with respect to the level of reception of the desired wave, that is, the SIR, is measured by the mobile station and the measured SIR is compared with the target SIR. When the result of the comparison is that the measured SIR is determined to be larger than the target SIR, the mobile station sends transmission power control information instructing a reduction of the transmission power of the transmitted signal from the base transceiver station to the mobile station, while conversely when the result is that the measured SIR is judged to be smaller than the target SIR, it sends transmission power control information instructing an increase of the transmission power of the transmitted signal from the base transceiver station to the mobile station. The first and second base transceiver stations raise and lower the transmission power to the mobile station based on the transmission power control information received from the mobile station. In this way, the transmission power is controlled so that the SIR measured by the mobile station becomes constant regardless of the present location of the mobile station.
Summarizing the problem to be solved by the invention, in the control of the downlink transmission power during a soft handover by each base transceiver station, when a mobile station is temporarily hidden in the shadow of a building etc., the level of reception of the mobile station falls. Therefore, the mobile station sends transmission power control information instructing an increase of the transmission power to the base transceiver station in the middle of soft handover. The base transceiver stations work to improve the quality of the received signal at the mobile station by raising the transmission power in accordance with the transmission power control information received from the mobile station.
During soft handover, however, since one mobile station is communicating with a plurality of base transceiver stations, if information requesting an increase of transmission power is sent from the mobile station, the transmission powers from all of the plurality of base transceiver stations end up rising simultaneously by exactly the same amount. Therefore, an increase in the overall amount of power emitted in the CDMA mobile communications system is caused. Along with this, a reduction in the subscriber capacity is caused. Still further, interference with other mobile stations in the middle of communication around that mobile station is caused.